The present invention relates to a method of producing a porous material having open pores. More particularly, the invention is concerned with a method of producing a porous material having open pores from a mixture which comprises a main ingredient, a polyamide hardener, a filler and water, the main ingredient comprising a bisphenol-type epoxy resin added with one or more reactive diluents each having one or more epoxy rings per molecule and added in an amount of 18 to 50 wt% based on 100 wt% of the bisphenol-type epoxy resin.
Hitherto, for producing a porous material having open pores for use as a filtering medium, air diffusion medium, casting mold, carrier for catalyst and so forth, various methods have been proposed such as sintering of metal powder, sintering of powdered thermoplastic resin, sintering of inorganic powder, hydration setting of cement or the like, pressing or stamping of a mixture of thermosetting resin and filler, hardening of a resin liquid containing pore-forming agent followed by the removal of the pore-forming agent by dissolving, extraction or evaporation, use of a foaming agent, and polymerization for hardening of a W/O emulsion of, for example, a polyester resin followed by evaporation of water from the hardened mass.
These known methods of producing porous material having open pores, however, encountered one or more of the following problems in connection with the manufacturing process. First of all, it is to be pointed out that these known methods impractically limit or restrict the shape and size of the product. In addition, these methods often require a heat treatment at high temperature, as well as press work at high pressure. The method which makes use of the pore-forming agent require a step of heating or vacuum operation for the removal of the pore-forming agent by evaporation. Furthermore, these known methods have disadvantages in that it is difficult to control the pore size or pore diameter, or that the production steps involved therein are generally complicated and difficult to conduct.
In order to solve the aforementioned problems and to produce a large scale porour material having open pores and relatively complicated shape, an improved method has been proposed by Japanese Patent Publication No. 2464/1978 whereby a porous material having precise dimensions and open pores of desired diameter can be produced. This improved method disclosed by the prior patent publication comprises the steps of preparing an O/W emulsion slurry from a mixture containing a glycidyl-type epoxy resin, a polymeric fatty acid polyamide hardener, a filler and water, casting the slurry in a waterimpermeable mold, hardening the slurry while it contains water, and dehydrating the hardened mass, whereby the desired object is attained. U.S. Pat. No. 4,464,485 discloses a method of producing a porous material, which is free from the aforementioned problems and suited for the production of a porous material which may be used as a filtering medium, air diffusion medium or casting mold or so forth, by adjusting the diameter of open pores within the range of from 0.5 to 10 microns. In this known method, the desired object is attained by the use of a bisphenol-type epoxy resin in combination with a special hardener which is either (a) mixture of (i) an amide compound which is obtained through a reaction between a monomeric fatty acid and an ethyleneamine represented by the formula of H.sub.2 N--(cH.sub.2 --Ch.sub.2 --NH).sub.n H where "n" is 3 to 5, and (ii) a polymeric fatty acid polyamide obtained through a reaction of a polymeric fatty acid and the aforementioned ethyleneamine, or (b) a reaction mixture obtained by mixing reaction of the aforementioned monomeric fatty acid, the aforementioned polymeric fatty acid and the aforementioned ethyleneamine.
However, the porous material having open pores produced by the aforementioned method of casting an emulsion slurry comprising a mixture of a bisphenol-type epoxy resin, a polyamide hardener, a filler and water has a maximum wet bending strength of only about 60 to 70 kg/cm.sup.2. When the thus produced porous material is used as a filtering medium, the filtering pressure and back-washing pressure that are repeatedly applied thereto cause cracking since the strength of the filtering medium is low. In addition, since it is required to apply a high pressure in order to increase the filtering speed in consideration of economy, the porous material used as the filtering medium must have a high strength.
It has been tried to decrease the content of water in the slurry thereto to lower the porosity of the hardened porous material, and to increase the strength of the porous material. However, this leads to the following two disadvantages. The first disadvantage is that the viscosity of the slurry is increased as the content of water therein is decreased, with the result that a mold having a complicated shape cannot be fully filled with the slurry at the casting step. The second disadvantage is that the reduction in porosity results in reduction of flow rate of a fluid flowing through the porous material so that an material becomes inferior from the economical point of view.